1. Field of the Invention
This invention relates to the field of polyamines.
2. Description of the Prior Art
The process of producing aromatic polyamines by the reaction of aniline and formaldehyde is well known and described for example in U.S. Pat. Nos. 2,683,730; 3,277,173; 3,344,162; and 3,362,979. By phosgenating these amines the corresponding isocyanates are obtained. The polyamines produced by the condensation of aniline and formaldehyde usually consist of a mixture of poly(methylenephenylamines) of functionality greater than two and the 2,2', 2,4' and 4,4' isomers of diamino-diphenylmethane. By reaction with phosgene a corresponding mixture of polyisocyanates and diisocyanates is prepared which is useful in producing, for example, polyurethane foam or is used as an epoxy curing agent.
One mode of reacting aniline with formaldehyde is to effect this reaction in the presence of a strong mineral acid, such as hydrochloric acid. Here a reaction occurs between the corresponding aniline hydrochloride and formaldehyde to provide a reaction mixture which, upon neutralization with a base, may be treated to recover the polyphenylamines. This process has left much to be desired. For example, it is necessary to utilize large quantities of both a mineral acid and a base which adversely affect the economics of the process and also the ease of conducting the reaction. In addition, the use of large quantities of mineral acids and the bases presents a severe corrosion problem. Also, the inorganic salt formed poses environmental difficulties with respect to disposal and/or recovery.
As an improvement to the conventional mineral acid catalyzed aniline-formaldehyde condensation, use of a solid acidic siliceous catalyst has been proposed (see U.S. Pat. No. 3,362,979). This is economically favorable over the conventional hydrochloric acid catalyzed process, since use of large quantities of corrosive acid and caustic are avoided. However, even this process has some drawbacks, particularly, in that the rate of reaction is not as rapid as desired and rearrangement of product amines at conventional conditions is not considered sufficiently complete.
Another acid catalyst utilized in the above reaction is methane sulfonic acid. However, this acid as well as hydrochloric acid forms insoluble salts of aniline in the initial stage of the reaction creating a problem of solids build-up and fouling of reactor stirring equipment. Also, there exists a problem in that both excess hydrochloric acid and methane sulfonic acid remaining after reaction are difficult to recover by water extraction. Lastly, when used as a recycle catalyst, the amine salts of methane sulfonic acid are usually solids at room temperature when the recycle catalyst is concentrated causing handling difficulties.